This invention relates to a clutch used, for example, in an image forming apparatus such as a copying machine, and in particular, to an electromagnetic clutch.
In recent years, the tendency to give a sufficient consideration to environment is promoted worldwide. Such tendency extends to so called OA equipment used in a office, and it is required that the OA equipment be energy-saving, reduce-exhaust such as dust and operate ozone, and quietly
Regarding one of the above-mentioned requirements, making the OA equipment quiet, it is possible to cope with it by providing a sound-insulating wall around the whole OA equipment. However, it is not desirable to provide such sound insulating wall because it increase both the manufacturing cost and the weight of the machines.
In order to make the operation of the OA equipment quiet without using the sound-insulating wall it becomes necessary to eliminate the cause of the noises thoroughly.
Incidentally, in an image forming apparatus such as a copying machine and a printer, which are a part of the OA equipment, a driving force transmitting system including a gear train and a timing belt mechanism is employed for transmitting a driving force, for example, to paper feeding rollers at a predetermined timing.
Such a driving force transmitting system is disposed in a manner to link a motor and the paper feeding rollers, and at the time of transmitting the driving force of the motor to the paper feeding rollers, the timing for paper feeding is determined by using an electromagnetic clutch disposed in the above-mentioned driving force transmitting system.
Now, the electromagnetic clutch according to the conventional technology will be explained. FIGS. 18 and 19 are partial cross-sectional views in the direction of the axial line of the electromagnetic clutch 910 according to the conventional technology; in FIG. 18, the electromagnetic clutch 910 is in the state of being off the driving force, and in FIG. 19 it is in the state of driving force transmitting.
In the state shown in FIG. 18, because an electric current does not flow in the coil 912, the armature 917 made of a magnetic material is retracted to the side of the gear 919 by the resilient force of the plate-shaped spring member 918. Owing to this, the rotor 915 and the armature 917 are kept in a completely separated state so that they may be prevented from rotating together to intercept the driving force transmission.
On the other hand, in the state shown in FIG. 19, because an electric current flows in the coil 912, the armature 917 is attracted to the rotor 915 by the generated electromagnetic force. Owing to this, the rotor 915 and the armature 917 are kept in a completely engaged state with each other to enable the driving force transmission.
As is clearly understood by FIG. 19, when the electromagnetic clutch 910 is switched to the transmitting state of driving force from the off state of driving force, the armature 917 is displaced for a distance D; at this time the armature 917 is accelerated to collide with the rotor 915, then the armature and the rotor vibrate to generate a comparatively large shock noise.
Further, when the electromagnetic clutch 910 is switched to the off state of driving force from the transmitting state of driving force, the armature 917 is retracted for the same distance D to collide with the spring member 918, or with the gear 919 through the spring member, thereby the armature 917, the rotor 915, and the gear 919 vibrate to generate a comparatively large shock noise in the same manner as the above.
As a countermeasure to reduce or eliminate the above-mentioned shock noise, it is considered to suppress the acceleration of the armature 917 to displace in the axial direction by making a control to rise the attracting force of the electromagnetic clutch 910 slowly. However, by such a countermeasure, in addition to the increase of the manufacturing cost due to the addition of the control circuit, the electromagnetic clutch is forced to be in the so called `half clutching` state, possibly to bring about the early abrasion of the electromagnetic clutch.
Incidentally, among the gears, timing belt pulleys, and so forth in the medium of driving force transmission, there are provided clearances to some extent. On the other hand, when the electromagnetic clutch is in the off state of driving force, the portion from the motor to the coupling portion to the driving source in the electromagnetic clutch rotates with a light load, but the paper feeding rollers are in contact with the paper sheet whereon an image is to be recorded and are in the state of being loaded.
When the electromagnetic clutch is brought into the transmitting state of driving force from the above-mentioned state, the coupling portion to the driving source and the coupling portion to the driven member are coupled in a moment to generate a shock noise. In the case where a gear train and so forth are provided in the downstream side of the electromagnetic clutch, such a shock noise is thought to be generated, due to the cause that the gears and the timing belt pulley in the upstream side run freely for the clearance and collide with the gears and toothed surface of the belt in the state of being loaded, at the moment when the electromagnetic clutch is brought into the transmitting state of driving force.
Further, in the case where a gear train and so forth are provided in the upstream side of the electromagnetic clutch, such a noise is thought to be generated due to the following cause. When the electromagnetic clutch is in the off state of driving force, the gears in the gear train are rotating by the driving force of the motor, but only a comparatively low rotating torque (several kg-cm) is transmitted because no load is given to it. In this state, when the electromagnetic clutch is brought into the transmitting state of driving force, a heavy load is suddenly given to the gear train and so forth; hence, a strong and unstable force is generated momentarily to act on the every tooth surface of the gears and the nip surface of the paper feeding rollers, and further, on the bearings inserted in the gears, the bearings of the rotating shaft of the gears, and so forth, causing the behavior of the every member to be put in disorder to generate a shock noise. In addition, in copying machines and printers of electrophotographic type, the torque of the paper feeding rollers is in the range from 5 kg-cm to 20 kg-cm. From the above-mentioned point of view to make the operation of OA equipment quiet, it is favorable to reduce or eliminate such a noise.
As a countermeasure for reducing or eliminating the above-mentioned shock noise, it is thinkable that the attracting force of the electromagnetic clutch is controlled to rise up slowly to rotate the driven portion gradually. However, by such a countermeasure, in addition to the increase of manufacturing cost due to the addition of the control circuit, the electromagnetic clutch is forced to be in the so-called `half-clutching` state for a long time, possibly to bring about the early abrasion in the facing of the electromagnetic clutch.